During many manufacturing and repair processes, two pieces of material are joined together using various techniques, including welding, brazing, riveting, and bolting. Riveting is often a preferred technique because it is generally low cost and lightweight. During a typical riveting process, a rivet is placed through a hole drilled through the two materials being joined. The rivet is then expanded, filling the hole. As the rivet expands, it contacts the sides of the hole in each of the materials, applying a radial force to the material. A protruding end or tail of the rivet can also be deformed to hold the rivet in place. Various rivets have been designed over the years, including high-strength hardened rivets having soft tail portions, which can be easily deformed without cracking (as described in U.S. Pat. No. 3,626,531).
When a composite material is riveted to a metallic material (e.g., during the manufacture of an aircraft), the radial force applied to the sides of the hole in the composite material can cause a compression of the composite fibers, damaging the composite and/or leading to delamination. To address this problem, the hole in the metallic material can be made small enough to allow the expanding rivet to contact the metallic sides of the hole (as described above) and the hole in the composite material can be made large enough so that the expanding rivet does not contact the composite sides of the hole, thus avoiding damage to the composite material. While this technique provides a lightweight method of joining the two materials, the requirement to drill two different size holes makes the process time consuming and expensive.
FIG. 1 illustrates an alternative method for fastening a composite material 1 and metallic material 2 with a bolting process. The bolting process allows a single size hole to be drilled through both materials 1, 2. The bolt 3 is placed through the hole and a nut 4 is coupled to the bolt 3. The nut 4 and bolt 3 can grip the materials 1, 2 without expanding into the local composite material 1. The bolt 3, however, is both heavier and more expensive to produce than a rivet. Additionally, while the bolting process is less labor intensive than riveting materials having different hole sizes, the bolting process is more labor intensive than the typical riveting process.